There are so many things you can talk about with this demonstration: solubility, density, immiscibility, etc. It also makes a good How Does That Work demonstration.
You have three bottles, each made of two layers, but in one bottle the layers are reversed. The bottles are filled with water (on the bottom) and baby oil (on the top). In two of the bottles, the water is colored with water soluble food dye (which will color water but not oil). In the third bottle, the oil is colored with liquid candle dye (which will color oil, but not water).
3 (500mL) water bottles
750 mL baby oil
750 mL water
Fill 2 of the bottles with 250 mL of water. Fill the remaining bottle with 250 mL of baby oil.
Use the candle dye to color the baby oil (do this before coloring the water - the water and food coloring are much more forgiving and easier to dispose of should you need to start over).
Use the food coloring to color the water, attempting to match the oil color as best you can. Keep track of what you used and repeat with the second water bottle.
Into the bottle with the oil, add 250 mL of water.
Into the two bottles with water, add 250 mL of baby oil to each. Cap. (You may wish to run some glue along the cap so they are more resistant to being opened).
Leave the bottles on your front table/desk and let the students explore. They'll try to turn the "wrong" one upside down. They may try to shake them and then watch them separate.
Should lead to some good discussions!
You can keep these forever - put them in a safe spot until you need them the next time!
*****************How Does That Work is a series of products and demonstrations that you can present to your students and challenge them to explain the science of how they work. Make sure you decide ahead of time what you'll accept as a valid explanation - can it be printed straight off the internet, written in the student's own words, or does the student need to be able to explain it to you conversationally? What will a valid explanation earn the student - a prize, extra credit, a feeling of goodness?